The present invention relates to vapor deposition techniques for material growth and in particular relates to chemical vapor deposition growth of epitaxial layers of semiconductor materials.
The term “vapor deposition” is used to refer to a number of techniques in which gas phase precursors condense or react to form a layer of a desired material on a substrate. Similarly, the phrase “chemical vapor deposition” (“CVD”) is often used to refer to a growth technique in which gas compositions react on a substrate surface to produce a desired product in the form of a high purity; high crystal quality epitaxial layer. Such layers are required for a number of semiconductor devices including, but not limited to, light emitting diodes (“LEDs”).
The physical and chemical characteristics of many semiconductor materials requires that their growth using CVD or related techniques be carried out at relatively high temperatures. For example, semiconductors such as gallium nitride (GaN) and associated Group III nitrides typically require CVD growth temperatures of at least about 600° C. Accordingly, the equipment used to carried out epitaxial growth must be able to generate, control and withstand such temperatures.
In relatively broad (but not limiting terms) a CVD reactor for epitaxial semiconductor growth typically includes a reaction chamber that can be evacuated to produce and control low gas pressures; a heating system; an inlet for reactant gases and an outlet for product gases; and some physical support (often a rotating support) for the substrate wafer(s) or wafer carrier(s) upon which growth will take place.
Several broad categories of heating techniques and systems are typically used for chemical vapor deposition. These include, but are not limited to, radio frequency (induction) heating; radiative heating; and resistive heating. The present invention relates to resistive heating.
Resistive heating is carried out by passing a current through a conductive filament, typically formed of particular metals, alloys, or other satisfactory materials (e.g. graphite). Although conductive, the filament is also selected so that a sufficient voltage applied to it will generate a current that heats the filament to relatively high temperatures. The filament will in turn heat the reactor, the growth substrate, and the reactants to the desired or necessary deposition temperatures.
Each of these heating techniques has inherent advantages and disadvantages. Ease of application is one advantage provided by resistive heating. The resistive element—i.e., the filament—is placed in close proximity to the sample to be heated (typically a semiconductor wafer or a carrier for semiconductor wafers). Direct or alternating current is applied to the filament and, as noted above, the element becomes hot as a function of the resistance of the filament and the current flowing through it. As the filament increases in temperature, it in turn heats the sample based upon the temperature of the filament and the distance between the filament and the sample.
Nevertheless, resistive heating also presents some inherent disadvantages. First, the filaments tend to expand at the high temperatures typically required for CVD. Such expansion frequently leads the filament to become distorted because of the difference in thermal properties (including coefficient of thermal expansion) between the filament and the various other components in the reactor. Such deformation can change the characteristics of the reactor's behavior from run to run and can reduce the lifetime of the heating assembly.
Second, resistive heating often creates large thermal gradients across portions of the reactor assembly. For example, in resistive heated deposition reactors, temperature changes of 500° C. or higher can occur across distances as small as 3 inches (75 mm). These large gradients can lead to cracking in the components.
Third, the electrodes that carry current to the filament are typically exposed to the growth environment and thus to the reactants, products, and by-products at high temperatures. As a result, undesired materials can accumulate on the electrodes and the electrodes can become electrically shorted to one another or to other components of the reactor.
Fourth, in some types of systems, the filaments are permitted to move inside of the growth apparatus as they expand and can form undesired electrical shunts (short circuits) with other components in the reactor. Additionally, such expansion can cause undesired contact between different portions of the same filament. Although not necessarily causing a short circuit, such filament self-contact can change the current flow (and thus the temperature) though the filament and lead to loss of temperature control or early degradation. These problems can become exacerbated when, as is typical in many reactors, more than one filament is used.
As another factor, the number of components required in a resistive heating assembly is relatively high providing a corresponding set of opportunities for component failure and overall difficulty.
As a specific example, resistive-heated chemical vapor deposition is one technique used to grow epitaxial layers of Group III nitrides on silicon carbide (SiC) substrates. In turn, such multiple epitaxial layers of Group III nitrides form the basis for light emitting diodes and diode lasers that, because of the wide bandgap of the Group III nitrides, can produce high frequency emissions in the blue, violet and ultraviolet portions of the electromagnetic spectrum. In turn, the ability to produce light at these frequencies offers the further opportunity to either drive phosphors that will emit white light or to combine blue emitting diodes with those emitting red and green light to produce white light within the visible spectrum.
In typical (but not limiting) techniques for growing Group III nitrides layers, a CVD reactor is typically used for between about four and five hours at a time to produce the desired layers of epitaxial growth. When one set is complete, the wafers or wafer carriers are removed from the chamber and are replaced with the next set of wafers upon which deposition is to be carried out. Each such cycle is commonly referred to as a “run,” and in conventional resistive-heated deposition reactors, between about 20 and 100 runs can be carried out before the chamber and its components must be cleaned, replaced, or both. Ordinary cleaning takes at least about two hours and more complex maintenance, considerably longer. The cumulative problems in resistive heating that have been noted herein tend to increase the frequency with which such maintenance or repair must take place.
As in any production technique, of course, reducing the frequency of disassembling, cleaning, or maintaining equipment corresponds to an increase in productivity and efficiency.
Accordingly, increasing the efficiency and throughput of such vapor deposition systems, and decreasing the frequency of maintenance and downtime, while maintaining the advantages of resistance heating, remains a worthwhile and desired goal.